For operation of wind energy installations, it is best for the wind conditions to be constant and for the wind to arrive at the wind energy installation as parallel to the rotor axis as possible. In practice, such ideal conditions frequently do not occur, and continual changes in the wind conditions must be coped with. From experience, a particularly high load for wind energy installations occurs when the wind arrives at the wind energy installation obliquely. Depending on the angular position, the rotor blades are then subject to different loads during revolution. This causes vibration, which can be transmitted from the rotor blades via the rotor shaft into the foundation of the wind energy installation.
DE 10 2006 034 106 A1 discloses a method in which the rotor rotation speed is reduced when the angle of the oblique incident flow, that is to say the angle between the instantaneous wind direction and the rotor axis, becomes too great. This method can admittedly in principle contribute to reducing the load on the wind energy installation. However, because of the very simple criterion on which the reduction of the rotation speed is made dependent, the rotor rotation speed is often also reduced when the wind energy installation is not actually subject to any particular load. This leads to unnecessary yield losses.